The proposed research focuses on a single early event in hormone action: the hormone induced nucleotide exchange reaction. First the reaction is identified as an important early reaction in the activation of adenylate cyclase by hormone. The proposed research then defines the kinetic details of the reaction sequence, probes the subunit organization responsible for these kinetic details, and identifies higher order membrane bound structures of the proteins involved in determining the rate of exchange process. Kinetic analysis includes (a) resolution of discrete steps within the overall reaction sequence, (b) the ligand requirements for each step, (c) the influence of cholera toxin and other chemical modification on the rates of each step, (d) the activation energy profile for the reaction sequence, and (e) the role of lipids in determining protein-protein interactions within the exchange process. Assignment of the kinetic steps to individual subunits is attempted by identifying the unit responsible for these steps, reconstituting it into acceptor membranes, and utilizing factors from systems displaying rapid exchange to correct the slow exchange process in the avian erythrocyte membrane. Higher order structures in systems of slow and rapid exchange are examined using the techniques of target analysis and nearest neighbor analysis. Considered together these experiments should provide a detailed kinetic and molecular description of this important early step in hormone action.